1. Field of the Invention
The invention relates to a control system for a hybrid vehicle including an engine and at least two motors or motor generators as driving force sources.
2. Description of Related Art
Japanese Patent Application Publication No. 8-295140 (JP 8-295140 A) and Japanese Patent Application Publication No. 2014-848 (JP 2014-848 A) describe a hybrid vehicle including an engine and two motor generators as driving force sources. In the hybrid vehicle, the engine and the first motor generator are respectively coupled to two rotating elements of a power split mechanism formed of a single-pinion-type planetary gear mechanism, and the second motor generator is coupled to a gear train coupled to drive wheels and an output element of the power split mechanism. The thus configured hybrid vehicle is allowed to select one of a hybrid mode and a one-motor mode. In the hybrid mode, the hybrid vehicle travels by transmitting the output torque of the engine to the drive wheels as a result of outputting reaction torque from the first motor generator. In the one-motor mode, the hybrid vehicle travels by transmitting the output torque of the second motor generator to the drive wheels in a state where the engine is stopped.
In addition, the hybrid vehicle described in JP 8-295140 A is configured to be able to transmit the output torque of the first motor generator to the drive wheels. Specifically, fixing means, such as a one-way clutch and a brake, for stopping an output shaft of the engine is provided. By stopping the output shaft of the engine with the use of the fixing means in this way, it is possible to transmit the output torque of the first motor generator to the drive wheels. Therefore, even when a required driving force is relatively high, the hybrid vehicle is able to travel by transmitting the output torque of both the first motor generator and the second motor generator to the drive wheels without driving the engine, so it is possible to suppress deterioration of fuel economy.
The hybrid vehicle described in JP 2014-848 A includes a mechanical oil pump that rotates integrally with the engine and an electric oil pump that is driven by energization and that has a smaller capacity than the mechanical oil pump. The hybrid vehicle is configured to, even when a drive mode that is determined on the basis of a required driving force and a vehicle speed is the one-motor mode but when the viscosity of oil is high, change the drive mode to the hybrid mode. This is because a load on the electric oil pump increases as the viscosity of oil becomes higher and there is a possibility that the durability of the electric oil pump decreases.
The oil pumps described in JP 2014-848 A may be mounted on the hybrid vehicle described in JP 8-295140 A. Because a torque that acts on a differential mechanism at the time when the two-motor mode in which a driving torque is output from each of the motor generators is selected is larger than a torque that acts on the differential mechanism at the time when the one-motor mode is selected, oil is supplied from the electric oil pump to the differential mechanism when the two-motor mode is selected. On the other hand, there is an inevitable delay from when driving of the electric oil pumps is started to when oil reaches the differential mechanism. Therefore, when driving of the electric oil pump is started simultaneously with switching of the drive mode to the two-motor mode, the differential mechanism transmits torque in a non-lubricated state or non-cooled state until oil reaches the differential mechanism, so there is a possibility of a decrease in durability. Alternatively, when the vehicle is caused to travel while another drive mode is set in order to suppress a decrease in the durability of the differential mechanism in this way, there is a possibility that a period during which the hybrid vehicle is allowed to be caused to travel in the two-motor mode shortens or an opportunity to cause the hybrid vehicle to travel in the two-motor mode reduces.